why apple repair has worsen my mac?

MY macbook pro late 2011 has been repaired on march 2015 for the notorious gpu failure.

After 1 month few and spiradic red artifact appeared. Now my mac cant boot, black screen with only 3 beeps and then poweroffs. (not a ram problem, I switched 3 different brands of ram)

why apple repair messed up the whole logic board ? I wont pay for a repair that after few months has killed my mac! with the sole gpu failire problem it was usable removing ati/amd kexts, now it is a piace of decor.